property Thumbnail;
Returns the raw bytes of the /Thumb entry for the current page (optional precomputed preview, typically a 64x64 raster).
Empty bytes when the page has no thumbnail attached.
Wraps the corresponding PDFium API and exposes it as a TPdf instance member so consumers do not have to manage FPDF_DOCUMENT handles directly.
Pdf1.Active before calling page-scoped operations; methods that require an open document raise EPdfError otherwise.SaveToFile to persist.
if Pdf1.Active then
Pdf1.Thumbnail;